Switching system for selectively enabling electrical power to be applied to plural loads

ABSTRACT

There is described a semiconductor switch device having two semiconductor components, the first component receiving drive signals from the second. The first component contains either a single power bipolar switch element for single ended control of the energisation of a load or a bridge of such elements enabling energisation of a load with different polarities. The switch elements, which may be transistors or darlington multiple transistors, have voltage dropping elements connected in the leads to their control bases, so that the drive signals from the second component can be produced by low impedance connection to one or the other supply conductor, thereby keeping the power dissipation of the second component to a low value. The voltage dropping elements may be a series resistor or a constant current circuit. The first component may have a temperature sensitive member, such as a diode, for diverting current from the control base of the or each switching element to terminate its conduction if the temperature of the component becomes too high. The switch device is used in a switching system for a motor vehicle, for example, where central control is required of a plurality of loads at various locations. A central controller transmits coded control signals to local controllers which direct the control signals to the correct switch devices. Status signals representing the functional states of the loads and the outputs of local sensors are sent to the central controller to produce there a corresponding display.

This is a division of application Ser. No. 164,366, filed Mar. 4, 1988,now U.S. Pat. No. 4,977,476 issued Dec. 11, 1990.

This invention relates to semiconductor switches.

Traditionally the feeding of electrical power to ancillary devices of amotor vehicle such as lamps for marking the boundaries of the vehicle,lighting the road ahead or indicating an intention to move to the leftor to the right, or electric motors for driving a windscreen wiper,fans, fluid pumps or raising and lowering windows, for example, has beencontrolled by switches near the steering wheel connected to the devicesby individual wires of sufficient thickness to carry the currentsinvolved. The cost of such wiring and switches is a major component inthe overall cost of the vehicle and their total weight is considerable.Moreover the mechanical switches can be unreliable because of contactfouling and erosion.

The need for a loom of wires and switches capable of carrying thecurrents can be avoided by the use of relays local to the devices but acomplex wiring loom is still required for the operating signals for therelays and the relays are still subject to wear, such as contacterosion.

In order to eliminate the need for the wiring loom and to improve thereliability of the system it has recently been proposed to provideperhaps only one current carrying conductor for all the ancillarydevices, which are connected to the conductor through individual localswitches and to multiplex control signals for these switches on to asignalling conductor. The switches have decoding circuits for detectingthe control signals addressed to them and power switches forimplementing the control required. Although relays could be used in suchmultiplexed control systems for switching the power to the loads, it ispreferable to use semiconductor switches because not only are they morereliable but also they enable the functioning of the system and the loaditself to be monitored. Additionally the elimination of mechanicalcontacts means that the system is safer because no sparks are produced.

Various proposals for a semiconductor switch for such an applicationhave been made. Circuits composed of discrete devices have been tooexpensive for wide acceptance. Circuits using a single semiconductordie, though still expensive, would be cheaper in volume production buthave suffered from the disadvantage, arising from the use of a DMOSoutput stage, that they cannot achieve the low voltage drop when on,required with currents up to for instance 6 amps with a heat sinktemperature of up to 110° C.

It is an object of the present invention to provide a semiconductorswitch suitable for a multiplex wiring system for a motor vehicle inwhich the above disadvantages are overcome. Although the invention is ofparticular value in the motor vehicle application it can also be used ina wide range of other industrial and consumer application.

According to one aspect of the present invention there is provided

a semiconductor switch device responsive to a control signal selectivelyto pass current to a load and to block the passage of current to theload and including

a first semiconductor component containing one or more power bipolarsemiconductor switching elements each of vertical construction having acontrolled current path extending from one major face to the other of asemiconductor body and arranged for connection in series with the loadacross a power supply, and control input means connected to the controlbase of the or each power bipolar semiconductor switching element, and

a second semiconductor component containing means responsive to thecontrol signal selectively to produce drive signals which are applied tothe control input means of the first semiconductor component for causingthe power bipolar semiconductor switching element or elements toconduct,

wherein the control input means includes voltage dropping means formedin the control base region of the or each power bipolar semiconductorswitching element.

The power bipolar semiconductor switching element may be a transistor ora darlington multiple and may be made in different current handlingcapacities to suit different loads.

The first semiconductor component may include a single power bipolarsemiconductor switching element which is connected in series with theload to switch on and off the electric current through the load.Alternatively, the first semiconductor component may include a bridgecircuit of power bipolar semiconductor switching elements with the powersupply and the load connected so that not only can the electric currentfeed to the load be switched on and off but also its polarity can bereversed. The drive signals produced by the second semiconductorcomponent are arranged to provide the polarity of energisation of theload indicated by the control signals. The four switching elements eachwith its own voltage dropping means may be formed on separatesemiconductor bodies or in pairs on two semiconductor bodies. If allfour switching elements are formed on a single semiconductor body stepsmust be taken to isolate the collectors of one pair of elements from thecollectors of the other pair.

The voltage dropping means may consist of a series connected resistor orconstant current circuit in the base lead of the or each power bipolarsemiconductor conductor switching element

The second semiconductor component may be an integrated circuit ofrelatively small current handling capacity dissipating relatively littleheat since the drive signal for the power bipolar semiconductorswitching element can be derived from the power supply through atransistor or transistors in very low or very high resistance statesonly, because the voltage dropping means for determining the drivecurrent of the power switching element is provided in the firstsemiconductor component.

The second semiconductor component may be arranged to monitor thecurrent through the load and the voltage applied to it to detect, forexample, open- and short-circuit conditions of the load, and produce anoutput indication of such abnormal status of the load. Timing means maybe provided in the second semiconductor component to determine the timefor which an excessive current is drawn by the load, and to terminatethe drive current to the first semiconductor component if the excessivecurrent is drawn for more than a predetermined time.

The first semiconductor component may include a temperature sensor suchas a diode or Schottky diode integrated with the power bipolarsemiconductor switching element(s). The forward conduction voltage orthe reverse bias leakage current of such a diode would indicate thetemperature component. The sensor may be connected to divert currentfrom the base of the switching element to tend to terminate itsconduction if the temperature becomes too high.

The two semiconductor components may be assembled into separate packagesor a single package. Since most of the heat dissipation will be from thefirst component the packaging may be arranged so that this component isattachable to a heat sink, and no particular arrangements need be madeto dissipate the heat from the second component.

According to a second aspect of the invention there is provided

a semiconductor component having a power bipolar semiconductor switchingelement of vertical construction in a semiconductor body with acontrolled current path extending from one major face to the other ofthe semiconductor body and arranged for connection in series with a loadacross a power supply and a control input terminal connected to thecontrol base of the semiconductor switching element through voltagedropping means formed in the control base region of the semiconductorswitching element.

The semiconductor switching element may be a transistor or a darlingtonmultiple.

The voltage dropping means may be a resistor or a constant currentcircuit connected in a series path from the control input terminal tothe control base of the semiconductor switching element.

An over-temperature sensor may be provided in the semiconductor body todetect excessive current through the switching element for a period oftime. The sensor may be a diode or a Schottky diode of which either thevoltage across it in forward conduction or the reverse bias leakagecurrent is monitored. The sensor may be connected to divert current fromthe base of the switching element to terminate its conduction if thetemperature of the semiconductor body becomes too high.

According to a third aspect of the invention there is provided

a semiconductor component having a power bipolar semiconductor switchingelement of vertical construction in a semiconductor body with acontrolled current path extending from one major face to the other ofthe semiconductor body and arranged for connection in series with a loadacross a power supply, a control input terminal connected to the controlbase of the semiconductor switching element through voltage droppingmeans and an over-temperature sensor in the semiconductor bodyresponsive to excessive temperature of the semiconductor body to divertdrive current from the control base of the switching element andterminate its conduction if the temperature of the semiconductor bodybecomes too high.

The semiconductor switching element may be a transistor or a darlingtonmultiple.

The voltage dropping means may be a series resistor or a constantcurrent source circuit and may be formed in the control base region ofthe semiconductor body.

The overtemperature sensor may be either a bipolar diode or a Schottkydiode of which either the voltage across it in forward conduction or thereverse bias leakage current is used to control the diversion of thedrive current.

According to a further aspect of the invention there is provided

a switching system for enabling electrical power to be appliedselectively to a plurality of loads in which control signals aretransmitted from a central controller to a plurality of local switchesrespectively connected to the loads, each local switch including

a first semiconductor component containing one or more power bipolarsemiconductor switching elements each of vertical construction andhaving a controlled current path extending from one major face toanother of a semiconductor body arranged for connection in series withthe load across a power supply, and control input means connected to thecontrol base of the or each power bipolar semiconductor switchingelement through voltage dropping means formed in the control baseregions, and

a second semiconductor component containing means responsive to thecontrol signal selectively to produce drive signals which are applied tothe control input means of the first semiconductor component for causingthe power bipolar semiconductor switching element or elements toconduct.

The control signals may be transmitted from the central controller tothe local switches via a common control line to a plurality of localcontrollers each connected to one or more local switches via individuallines, the local controllers including decoding means responsive tocodes contained in the control signals to detect and forward the controlsignals to the local switches to which they are addressed.

The local switches may be arranged to generate status signals indicatingthe functional states of the loads to which they are connected, thestatus signals being returned to the control controller where a displayof the status, particularly if it is a fault, may be produced. Where alocal controller is provided it may additionally or alternatively beconnected to one or more local sensors and generate report signalsindicating the sensed conditions which are transmitted to the centralcontroller for display or monitoring.

The switching system may be used in a vehicle such as a motor vehicle,the central controller being arranged to respond to inputs from thedriver to produce control signals and to produce displays visible tohim.

In order that the invention may be fully understood and readily carriedinto effect, embodiments of it will now be described with reference tothe accompanying drawings, of which:

FIG. 1 is a diagram of an example of a system incorporating switchesaccording to an example of the invention;

FIG. 2 is a diagram of the switch used in FIG. 1;

FIG. 3 shows the circuit of a power transistor suitable for use in theswitch of FIG. 2 with a resistor integrated into the base lead producinga parasitic transistor;

FIG. 4 is a cross-sectional diagram of the transistor shown in FIG. 3;

FIG. 5 is the circuit of an alternative power transistor suitable foruse in the switch of FIG. 2 with a constant current source integratedinto the base lead in place of the resistor shown in FIG. 3;

FIG. 6 is a cross-sectional diagram of the transistor shown in FIG. 5;

FIG. 7 shows the circuit of a power transistor with one form ofover-temperature protection;

FIG. 8 shows the transistor of FIG. 5 with the over-temperatureprotection shown in FIG. 7;

FIG. 9 is a cross-sectional diagram of parts of the transistor shown inFIG. 8;

FIG. 10 is a diagram of an alternative form of switch for energising aload with currents of different polarities; and

FIG. 11 shows a modification to the switch shown in FIG. 10.

The system shown in FIG. 1 could be used in a motor vehicle or in anindustrial or domestic installation where remote control of theenergisation of a number of electrical devices is required. A centralcontroller 1 is connected to receive inputs from a unit 2, which may,for example, comprise a keyboard or a plurality of manually operableswitches, and supplies outputs to indicators and displays 3, including,for example, warning lamps, light-emitting diode and/or liquid crystaldisplays, and digital to analogue converters connected to meters.

The central controller 1 is connected via a bus line 4 to a plurality oflocal controllers of which only one is shown having the reference 5. Thelocal controller 5 is energised via power supply conductors 6 and 7, forexample connected to the motor vehicle battery, and these conductorswith the bus line 4 may conveniently be formed into a single cable whichis routed about the vehicle as required. Alternatively the earth returnmay be through the body-chassis structure of the vehicle in which casethe earthed conductor may be omitted. The local controller 5 isconnected via individual conductor pairs 8,9 to local switches of whichonly 2 are shown having the references 11 and 12. The local controller 5is also connected via conductors 13 to receive inputs from local sensorsof which only one is shown.

Each local switch, 11 or 12, includes a linear chip 15 to which theconductor pair 8,9 is connected, a power chip 16 and a resistor 17. Thepower chip 16 and the resistor 17 are connected in series with eachother and in series with a load 18, the energisation of which the localswitch is to control, across the power supply conductors 6 and 7. Thelinear chip 15, which is also connected to the power supply conductors 6and 7, is connected to control the power chip 16 via a conductor 19 andhas two other leads 20 and 21 connected respectively to the endterminals of the resistor 17.

The central controller 1, which may be constructed as a conventionalmicrocomputer having a microprocessor connected by data and addressbuses to random access memory, read-only memory and input-outputcircuitry, repeatedly scans the switches in the input unit 2 and reactswhenever there is a change in the state of the switch. The status of theswitches are stored in the random access memory of the controller 1, anddepending on the change of state of the switches in the unit 2, thecontroller 1 generates coded control signals representing the changes ofenergisation, off or on, required for the various loads 18. These codedcontrol signals are multiplexed onto the conductor 4 and each includes acode indentifying to which of the loads 18 the control signal isaddressed.

A local controller 5, which may also include a conventionalmicrocomputer, detects those of the coded control signals which areaddressed to loads under its command and records the energisation statusof those loads, changing the status in response to the coded controlsignals as they are received. The local controller 5 generates an outputon the conductor 9 which is connected to the local switch, say 11,controlling the energisation of the load 18 to which the change relates.In the local switch the linear chip 15 contains a multivibrator thestate of which is set or reset depending on whether or not thecorresponding load 18 is to be energised or not. The linear chip 15provides a drive signal via the conductor 19 to switch on and off apower transistor included in the power chip 16, thereby switching theenergisation of the load 18 on and off. The resistor 17 included inseries with the load 18 will develop a voltage due to the currentthrough the load, which voltage is conveyed by the conductors 20 and 21to the linear chip 15. In addition, the voltage across the load 18 isalso provided by the conductor 21. The linear chip 15 contains circuitryfor detecting short and open circuit conditions of the load 18 and asignal representing these conditions is conveyed by the conductor 8 backto the local controller 5. Preferably, the signal on the conductor 8from the linear chip 15 which represents satisfactory operation of theload is an active one so that should the local switch fail an indicationof its failure will be conveyed to the local controller 5.

The status information obtained from the local switches together withdata from local sensors connected to the controller 5 are stored in itand communicated to the central controller 1 between the multiplexedcoded control signals emitted by the controller 1. Typically, a localsensor may include a variable resistor the resistance of which iscontrolled by the quantity being sensed, and an analogue to digitalconverter is used to change a voltage proportional to that resistance toa digital value which is used by the local controller 5. The indicatorsand displays 3 are energised by the central controller 1 to provideinformation to the driver of the quantities sensed by the various localsensors, the energisation of the various loads 18 and whether any ofthose loads has failed.

FIG. 2 shows in some more detail the construction of a local switch andin this figure the reference numerals used correspond to those used inFIG. 1.

As shown in FIG. 2, a power chip 16 includes a PNP power transistor 25and a resistor 26 connected in series in its base connection. It is afeature of the present invention that the local switch consists of twosemiconductor dies, the linear chip 15 and the power chip 16, which maybe assembled into two packages or combined into a single package. Asmentioned above, the linear chip contains all the logic to communicatewith the central controller via the local controller, to detect faultconditions and to control the power chip. The power chip reacts to thecontrol signals from the linear chip and switches the current to theload 18. The PNP transistor 25 in the power chip is constructed so as tobe capable of handling the current required by the load 18 with asufficiently low voltage drop through its controlled current path whenon.

In a modification the PNP transistor may be replaced by a darlingtonpair or triple.

The PNP transistor 25 has the resistor 26 connected in series in itsbase lead, which is integrated into the semiconductor die on which thepower transistor is formed, to provide a voltage drop so that the drivesignal applied to the conductor 19 by the linear chip is connectedthrough low resistance switches either to the negative supply conductor6 or the positive supply conductor 7, so that power dissipation in thelinear chip 15 arising from the control of the drive current for thetransistor 25 is very low, and a major part of the heat dissipation ofthe switch is from the power chip 16 rather than the linear chip 15.This means that the heat sinking requirements of the local switch as awhole can be concentrated on the power chip 16, with very relaxedassembly requirements for the linear chip 15 which can therefore bemounted in a low cost package.

The use of a PNP transistor as a switch element of the power chipprovides an optimum solution for a motor vehicle environment due to itsvery low on-voltage capability. As mentioned above, the transistor couldbe replaced by a darlington multiple, such as a darlington pair, whichwould give a much greater gain and allow the switching of higher loadcurrents using a low drive current from the linear chip 15.

The use of a simple series resistor as the voltage dropping structureconnected in the base lead of the PNP transistor has the advantage thatit can be produced as part of the emitter diffusion process or by aseparate diffusion step during the fabrication of the transistor. Theresistor has the disadvantage that it results in a parasitic transistorbeing formed which can bypass a fraction of the base drive current whenthe PNP power transistor is in saturation. FIG. 3 is a circuit diagramshowing the parasitic transistor as T2 connected to the power transistorT1. The bypassing of the base drive current can be reduced by includingan extra heavy base diffusion around the region forming the resistor.This extra diffusion is shown in FIG. 4, which shows a diagrammaticcross-section of the PNP power transistor and the series resistorconnected in its base lead.

In FIG. 4, an epitaxial layer 31 of P-type conductivity is formed on thesurface of a P⁺ -conductivity substrate 30. A base region 32 of N-typeconductivity is formed in the epitaxial layer 31, and an emitter regionof P⁺ -type conductivity is formed in the base region. This constitutesthe PNP power transistor. The resistor is formed by a P⁺ -conductivityregion 34 in an extra heavy base diffusion region 35, itself within thebase region 32. As mentioned above, the extra heavy base region 35serves to reduce the gain of the parasitic inverse transistor shown asT2 in FIG. 3.

A constant current source circuit can be used instead of the simpleresistor in the base lead of the transistor switch element. FIG. 5 showssuch a circuit connected in series in the base lead of the powertransistor T1 and includes an NPN transistor T3 having its emitter leadconnected through a resistor R4 to a terminal A and having its collectorconnected to the base of the transistor T1. A series chain of resistorsR3 and two diodes D2 and D3 is also connected from the base of thetransistor T1 to the terminal A with the junction between the resistorR3 and the diode D2 being connected to the base of a transistor T3. Theoperation of the constant current circuit is conventional in that theforward conduction voltage of the diodes D2 and D3 provides a biasvoltage for the transistor T3 to conduct just sufficient current throughthe resistor R4 to maintain the transistor T3 in conduction. Althoughthe fabrication of the constant current source requires an extradiffusion step as compared with the fabrication of the simple resistorshown in FIG. 4, the use of a constant current source has the advantagethat the base current of the power transistor T1 is independent of thesupply voltage which in a motor vehicle can vary between 7 and 16 volts.Since the base current of the power transistor T1 is kept constant, thedrive current produced by the linear chip 15 can be kept more nearly tothe minimum necessary to operate the power transistor with the resultthat the size and cost of the linear chip can be considerably reduced.

FIG. 6 is a cross-sectional diagram of the circuit shown in FIG. 5formed in a semiconductor body. The parts of the structure whichconstitute the elements of the circuit shown in FIG. 5 are indicated inFIG. 6. The constant current source circuit also produces a parasitictransistor in a similar way to the resistor, and the effects of thisparasitic transistors are reduced as described above by the provision ofN⁺ regions containing the resistors and diodes of the circuit.

The power chip may be provided with internal self-protection againstexcessive current through it due to short-circuits, for example, and toexcessively high temperatures which could lead to damage to thesemiconductor devices. As mentioned above, the linear chip can be usedto detect short-circuiting of the load 18 by monitoring the voltage onthe conductor 21. Because of the nature of the loads in certaininstances, for example incandescent lamps or motors, the load itself mayappear as a short-circuit when first turned on and only reach its normalrunning resistance after a delay of a fraction of a second. It followstherefore that any protection against excessive current through thepower chip must have a built-in delay before conduction is terminated toprotect the semiconductor devices. To provide this delay the linear chipmay be provided with a timer which prevents any protective action beingtaken until an appropriate delay of, say, 0.3 seconds has elaspsedfollowing the start of an apparent short-circuit. If the voltage acrossthe load or the current through it has not reached an acceptable valuewithin the delay period, then the linear chip would be arranged toremove the drive current from the power chip to provide a controlledshut down of the circuit. The load current may alternatively bemonitored by measuring the voltage across the resistor 17 connected inseries with the load. This resistor need not be a discrete element or athin or thick film deposited layer, but may simply be a section ofprinted circuit board conductor which is already included in the system.

An alternative to measuring the excessive current after a delay time, isto integrate into the power chip an over-temperature sensor. The use ofsuch a sensor has the advantage that the necessary time delay isautomatically provided by the heat capacity of the chip itself and, inaddition, monitors the temperature of the chip which is a certainindicator of the likelihood of damage to the chip. In one example of theover-temperature sensor, a forward biased diode is provided on the powerchip and receives a small current from the linear chip. The voltage dropacross the diode is monitored by the linear chip to provide informationto that chip relating to the temperature of the power chip. In anotherexample, the diode on the power chip is reverse biased and the leakagecurrent from the diode is monitored by the linear chip to provideinformation about the temperature of the power chip. The sensitivity ofthe measurement provided by the latter example would be enhanced by theuse of a Schottky diode having a higher reverse bias leakage current. Adisadvantage of this arrangement is that it requires two additionalconnections from the linear chip to the power chip.

A further example which requires no additional connections from thelinear chip to the power chip involves providing a diode or othertemperature sensor structure on the power chip together with necessaryextra circuitry for diverting the base drive current of the powertransistor away from the base so as to starve the transistor of basedrive and turn it off. FIG. 7 shows the circuit of an example in whichthe leakage current of a Schottky diode D1 fed through a resistor R2from the base drive circuit for the power chip is amplified by adarlington pair consisting of transistors T4 and T5.

The use of an over-temperature sensor obviates the need for a timer onthe linear chip, thereby giving a considerable saving in die area andtherefore cost.

The over-temperature sensor just described may be combined with theconstant current circuit shown in FIG. 5, giving the circuit shown inFIG. 8. The individual circuit elements are shown in the cross-sectionof a semiconductor body in FIG. 9. To produce the circuit of FIG. 8 itis necessary to produce appropriate numbers of components required bythe circuit and interconnect them as shown in the figure.

As described above, the local switches are suitable for single endedloads with one terminal grounded. Many electric motors in motorvehicles, however, require switching of the direction of the currentflow to reverse the motor. An example of this is the use of a motor toopen and close the windows of the vehicle. Reversal of the current flowcould be achieved by the use of four semiconductor switching elements,such as transistors each with additional circuit components as describedabove, connected in a bridge configuration with the power supplyconnected across one diagonal of the bridge and the load connectedacross the other diagonal. Obviously, the linear chip controlling such apower chip or chips would have to be able to turn on diagonally oppositepairs of transistors depending on the direction of current flow requiredthrough the load. FIG. 10 is a diagram of a suitable circuit for such alocal switch. The transistors together with their base series resistorsor other circuits may be formed on separate chips or combined on one ortwo chips.

FIG. 11 shows a modification of FIG. 10 in which two transistors areconnected to be driven directly from the linear chip and the other twotransistors are driven by the output currents of the first twotransistors. This arrangement has the advantage of reducing thedissipation from the linear chip and also simplifying it.

Although the invention has been described with reference to specificexamples concerned with the particular problems encountered in motorvehicles, it will be appreciated that the same techniques would beapplicable to the energisation of a plurality of remotely located loadsfrom a central control position both in domestic and industrialenvironments.

What we claim is:
 1. A switching system for selectively enablingelectrical power to be applied to a plurality of loads, said switchingsystem comprising:a plurality of local switches respectively connectedto a corresponding one of said plurality of loads, each of saidplurality of local switches including first semiconductor componentmeans comprising at least one power bipolar semiconductor switchingelement, said power bipolar semiconductor switching element having asemiconductor body of vertical construction provided with top and bottommajor faces and having a controlled current path extending from onemajor face to the other major face, the corresponding one of saidplurality of loads being connected in series to the semiconductor bodyof said power bipolar semiconductor switching element and across a powersupply, said power bipolar semiconductor switching element including abase, an emitter and a collector, control input means connected to thebase of said power bipolar semiconductor switching element and includingvoltage dropping means, second semiconductor component means connectedto said control input means and including means responsive to a controlsignal selectively to produce output drive signals applied to saidcontrol input means of said first semiconductor component means foractivating said power bipolar semiconductor switching element to beconductive, resistor means having first and second terminals at itsopposite ends respectively connected to said power bipolar semiconductorswitching element and to said load corresponding thereto, and first andsecond conductors connected to said second semiconductor component meansand to said first and second terminals at the opposite ends of saidresistor means for providing voltage data from said resistor means tosaid second semiconductor component means in response to the occurrenceof current through said corresponding load; and central controller meansoperably connected to each of said plurality of local switches forselectively transmitting control signals to respective secondsemiconductor component means of selected local switches and to whichsaid second semiconductor component means of the corresponding localswitch is responsive; each of said power bipolar semiconductor switchingelements of a respective one of said plurality of local switchesswitching from a non-conductive state to a conductive state forproviding electric current to the load corresponding thereto in responseto an output drive signal from said second semiconductor component meansapplied to said control input means conditioned upon a control signalfrom said central controller means being selectively applied to saidsecond semiconductor component means of respective local switches and towhich said second semiconductor component means of a selected localswitch is responsive.
 2. A switching system as set forth in claim 1,further including a plurality of local controller means, each of saidplurality of local controller means being connected to at least one ofsaid local switches;a common control line connecting said plurality oflocal controller means to said central controller means for receivingthe control signals as transmitted from said central controller means;and each of said local controller means including decoding meansresponsive to code contained in the control signals as transmitted fromsaid central controller means to detect and selectively transmit thecontrol signals to the respective local switches to which they areaddressed via the code contained in the control signals.
 3. A switchingsystem as set forth in claim 2, wherein respective ones of saidplurality of local controller means are connected to at least one localsensor, each of said local controller means connected to at least onelocal sensor being responsive thereto for generating report signalsindicative of the sensed conditions; andsaid central controller meansreceiving the report signals as generated by the respective localcontroller means for monitoring the sensed conditions indicated thereby.4. A system according to claim 2, wherein the local switches arearranged to generate status signals indicating the functional states ofthe loads to which they are connected.
 5. A system according to claim 4wherein the status signals are returned to said central controllermeans, the central controller means including means responsive to thestatus signals to produce a display representing the functional state ofa respective load.
 6. A switching system as set forth in claim 1,wherein said second semiconductor component means comprises a linearintegrated circuit having a relatively small current handling capacityand dissipating relatively little heat when energized.
 7. A switchingsystem as set forth in claim 1, wherein said second semiconductorcomponent means monitors current through the load corresponding theretoand voltage applied to the load via said first and second conductorsconnected to said first and second terminals at the opposite ends ofsaid resistor means for detecting open-circuit and short-circuitconditions of said load; andsaid second semiconductor component meansproducing an output signal in response to the detection of anopen-circuit or short-circuit condition of said load correspondingthereto indicative of the abnormal status of said load.
 8. A switchingsystem as set forth in claim 1, wherein said second semiconductorcomponent means further includes timing means for determining the timeduring which said corresponding load draws an excessive current; andsaidsecond semiconductor component means terminating the production of anoutput drive signal as applied to said control input means of said powerbipolar semiconductor switching element when the excessive current drawnby said corresponding load exceeds a predetermined time interval torender said power bipolar semiconductor switching element non-conductivefor discontinuing transmission of electric current to said correspondingload.
 9. A switching system for selectively enabling electrical power tobe applied to a plurality of loads, said switching system comprising:aplurality of local switches respectively connected to a correspondingone of said plurality of loads, each of said plurality of local switchesincluding first semiconductor component means comprising at least onepower bipolar semiconductor switching element, said power bipolarsemiconductor switching element including a semiconductor substrate ofone conductivity type, a first dopant region of the other conductivitytype in said semiconductor substrate of said one conductivity type andopening onto the top surface thereof, a second dopant region of said oneconductivity type disposed in said first dopant region of the otherconductivity type and opening onto the top surface thereof, a thirddopant region of the other conductivity type disposed in said firstdopant region of the other conductivity type and opening onto the topsurface thereof, said third dopant region of the other conductivity typehaving a higher dopant concentration than said first dopant region ofthe other conductivity type and being disposed in spaced relation tosaid second dopant region of said one conductivity type provided in saidfirst dopant region of the other conductivity type, a fourth dopantregion of said one conductivity type disposed in said third dopantregion of said other conductivity type of higher dopant concentration,said semiconductor substrate of said one conductivity type and saidfirst, second, third and fourth dopant regions defined therein forming avertically arranged bipolar transistor having said first dopant regionof the other conductivity type as the base, said second dopant region ofsaid one conductivity type as the emitter, and the bottom portion ofsaid substrate as the collector and having a controlled current pathextending from the top surface thereof to the bottom surface thereof,the corresponding one of said plurality of loads being connected inseries to the collector region of said bipolar transistor and across apower supply, said fourth dopant region of said one conductivity typedefining a control input means connected to the base of said bipolartransistor and providing voltage dropping means, second semiconductorcomponent means connected to said control input means and includingmeans responsive to a control signal selectively to produce output drivesignals applied to said control input means of said first semiconductorcomponent means for activating said power bipolar semiconductorswitching element to be conductive, resistor means having first andsecond terminals at its opposite ends respectively connected to saidpower bipolar semiconductor switching element and to said loadcorresponding thereto, and first and second conductors connected to saidsecond semiconductor component means and to said first and secondterminals at the opposite ends of said resistor means for providingvoltage data from said resistor means to said second semiconductorcomponent means in response to the occurrence of current through saidcorresponding load; and central controller means operably connected toeach of said plurality of local switches for selectively transmittingcontrol signals to respective second semiconductor component means ofselected local switches and to which said second semiconductor componentmeans of the corresponding local switch is responsive; each of saidpower bipolar semiconductor transistors of a respective one of saidplurality of local switches switching from a non-conductive state to aconductive state for providing electric current to the loadcorresponding thereto in response to an output drive signal from saidsecond semiconductor component means applied to said control input meansconditioned upon a control signal from said central controller meansbeing selectively applied to said second semiconductor component meansof respective local switches and to which said second semiconductorcomponent means of a selected local switch is responsive.
 10. Aswitching system as set forth in claim 9, further including a pluralityof local controller means, each of said plurality of local controllermeans being connected to at least one of said local switches;a commoncontrol line connecting said plurality of local controller means to saidcentral controller means for receiving the control signals astransmitted from said central controller means; and each of said localcontroller means including decoding means responsive to code containedin the control signals as transmitted from said central controller meansto detect and selectively transmit the control signals to the responsivelocal switches to which they are addressed via the code contained in thecontrol signals.
 11. A switching system as set forth in claim 9, whereinsaid second semiconductor component means comprises a linear integratedcircuit having a relatively small current handling capacity anddissipating relatively little heat when energized.
 12. A switchingsystem as set forth in claim 9, wherein said second semiconductorcomponent means monitors current through the load corresponding theretoand voltage applied to the load via said first and second conductorsconnected to said first and second terminals at the opposite ends ofsaid resistor means for detecting open-circuit and short-circuitconditions of said load; andsaid second semiconductor component meansproducing an output signal in response to the detection of anopen-circuit or short-circuit condition of said load correspondingthereto indicative of the abnormal status of said load.
 13. A switchingsystem as set forth in claim 9, wherein said second semiconductorcomponent means further includes timing means for determining the timeduring which said corresponding load draws an excessive current; andsaidsecond semiconductor component means terminating the production of anoutput drive signal as applied to said control input means of said powerbipolar semiconductor switching element when the excessive current drawnby said corresponding load exceeds a predetermined time interval torender said power bipolar semiconductor switching element non-conductivefor discontinuing transmission of electric current to said correspondingload.
 14. A switching system for selectively enabling electrical powerto be applied to a plurality of loads, said switching systemcomprising:a plurality of local switches respectively connected to acorresponding one of said plurality of loads, each of said plurality oflocal switches includingfirst semiconductor component means comprisingat least one power bipolar semiconductor switching element having asemiconductor body and including a base, an emitter and a collector, thecorresponding one of said plurality of loads being connected in seriesto the semiconductor body of said power bipolar semiconductor switchingelement and across a power supply, control input means connected to thebase of said power bipolar semiconductor switching element and includingvoltage dropping means, second semiconductor component means connectedto said control input means and including means responsive to a controlsignal selectively to produce output drive signals applied to saidcontrol input means of said first semiconductor component means foractivating said power bipolar semiconductor switching element to beconductive, resistor means having first and second terminals at itsopposite ends respectively connected to said power bipolar semiconductorswitching element and to said load corresponding thereto, and first andsecond conductors connected to said second semiconductor component meansand to said first and second terminals at the opposite ends of saidresistor means for providing voltage data from said resistor means tosaid second semiconductor component means in response to the occurrenceof current through said corresponding load; and central controller meansoperably connected to each of said plurality of local switches forselectively transmitting control signals to respective secondsemiconductor component means of selected local switches and to whichsaid second semiconductor component means of the corresponding localswitch is responsive; each of said power bipolar semiconductor elementsof a respective one of said plurality of local switches switching from anon-conductive state to a conductive state for providing electriccurrent to the load corresponding thereto in response to an output drivesignal from said second semiconductor component means applied to saidcontrol input means conditioned upon a control signal from said centralcontroller means being selectively applied to said second semiconductorcomponent means of respective local switches and to which said secondsemiconductor component means of a selected local switch is responsive.15. A switching system as set forth in claim 14, wherein said secondsemiconductor component means comprises a linear integrated circuithaving a relatively small current handling capacity and dissipatingrelatively little heat when energized.
 16. A switching system as setforth in claim 14, wherein said second semiconductor component meansmonitors current through the load corresponding thereto and voltageapplied to the load via said first and second conductors connected tosaid first and second terminals at the opposite ends of said resistormeans for detecting open-circuit and short-circuit conditions of saidload; andsaid second semiconductor component means producing an outputsignal in response to the detection of an open-circuit or short-circuitcondition of said load corresponding thereto indicative of the abnormalstatus of said load.
 17. A switching system as set forth in claim 14,wherein said second semiconductor component means further includestiming means for determining the time during which said correspondingload draws an excessive current; andsaid second semiconductor componentmeans terminating the production of an output drive signal as applied tosaid control input means of said power bipolar semiconductor switchingelement when the excessive current drawn by said corresponding loadexceeds a predetermined time interval to render said power bipolarsemiconductor switching element non-conductive for discontinuingtransmission of electric current to said corresponding load.